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Chapter 35 Review: Parallel and Series Circuits

35.1 A Battery and a Bulb

  • in order to light a bulb, there must be a complete circuit (or any unbroken path that an electron can flow) from the positive terminal (top of battery) to the negative terminal (bottom of battery)
Electrons flow through the filament of the light bulb
Electrons flow through the filament of the light bulb


http://www.mikecurtis.org.uk/images/bulb.gif

  • electrons flow from the bottom or negative end of the battery, through the wire to the bottom of the light bulb, through the filament inside the bulb, and then out through the other end of the wire to the positive end of the battery. the current passes through the inside components of the battery and completes the circuit
  • the electrons do not collect and gather in a specific place, they flow continuously in around a loop, or circuit and then through the battery

35.2 Electric Circuits


  • in order to have a continuous flow of electrons, there must be no gaps or breaking points in a circuit, or else the electrons will cease to flow
  • a gap is sometimes provided by an electrical switch that can be opened or closed depending on whether the circuit needs to be cut off or needs to allow electron
  • when an electrical switch is closed, current flows in the same way that water does when the faucet is opened. Hence, closing a circuit means that electrons are then allowed to flow. A circuit must be closed in order to allow for the flow of electrons
  • when a switch is opened, electrical flow is stopped completely
  • two types of circuits allow for more than one device to receive electric energy:
    • series or parallel circuits
  • when a circuit is in series, it forms a single pathway for electrons to flow between the terminals or battery
  • when a circuit is in parallel, it forms branches that each have a separate pathway for the flow of electrons
  • both connections have their own distinct features and circumstances

35.3 Series Circuits

  • in a series circuit, when the switch is closed a current exists almost immediately in the connection to the light bulbs
  • the current does not bunch up in certain places, but flows through each light bulb; the electrons begin to move at once from the negative end of the battery, through the bulb filaments, and to the positive terminal of the battery
  • the electrons eventually move all the way around the connection
  • any break in this path causes the circuit to open, and ceases the flow of electrons
  • such a break could possibly result from a broken filament or opening the switch to the circuit

3 light bulbs connected in a series circuit. The flow of electrons would cease in all three bulbs if the connection was to be broken
3 light bulbs connected in a series circuit. The flow of electrons would cease in all three bulbs if the connection was to be broken

http://www.smgaels.org/physics/home/hand_2/series.jpg

  • Important Characteristics for a Series Circuit:
    1. Electric current has but one pathway through the circuit. The current passing through the electrical device is the same.
    2. The current is resisted by the resistance of the first device, the second, the third etc., therefore, the total resistance to current in the circuit is the sum of the individual resistances along the path. This means that the sum of all the resistances in the three devices is the total amount of resistance in the circuit.
    3. The current in the circuit is numerically equal to the voltage supplied by the source divided by the total resistance of the circuit, or rather, Ohm's Law.
    4. Ohm's Law applies individually to each device. the voltage drop (potential difference) across each device depends directly on the resistance. This is so because it takes more energy to move a unit of charge through a large resistance versus a small resistance.
    5. The total voltage impressed across a series circuit divides among the individual electrical devices in the circuit so that the sum of the voltage drops across each individual device is equal to the total voltage supplied by the source.

  • a disadvantage of the series circuit would obviously be that if one device stops working, the current in the whole circuit ends and none of the devices will then function
  • such an example would be in a strand of Christmas lights; if one bulb goes out, you must replace the burned out bulb in order for the strand to work again

35.4 Parallel Circuits

  • a parallel circuit has its own separate path from one battery terminal to the other terminal.
  • in the below picture, there are three individual paths from each of the three bulbs to the battery, thus resulting in the constant flow of electrons to each resistor
  • the current in one of the bulbs does not pass through the other bulbs
  • the parallel circuit is always completed regaurdless of whether one , two, or all three bulbs are lit.
  • a break in one of the paths does not affect the flow of electrons in the other remaining bulbs
  • each bulb opperates independently of the other bulbs
3 lamps connected in a parallel circuit. Electrons still flow if one of the bulbs is cut off, because each bulb has its own separate connection to the electrical source
3 lamps connected in a parallel circuit. Electrons still flow if one of the bulbs is cut off, because each bulb has its own separate connection to the electrical source


http://www.berkeleypoint.com/images/parallel.jpg
  • Four Characteristics of a Parallel Circuit:
    1. The voltage is constant throughout the device.
    2. The amount of current in each branch in inversely proportional to the resistance of each branch. Rather, Ohms Law is applicable individually to each branch. The total current is divided among the three parallel branches of the device.
    3. The total current in the circuit equals the sum of the currents in its parallel branches.
    4. When the number of branches is increased, the total resistance decreases. Resistance in the circuit is reduced when more branches are added between any two points of the circuit. Therefore, the total resistance of the circuit is less than the resistance of any one of the separate branches.

35.5 Schematic Diagrams


Symbols for some of the common circuit components
Symbols for some of the common circuit components

http://www.teachengineering.org/collection/cub_/lessons/cub_images/cub_electricity_lesson06_fig5.jpg
  • schematic diagrams are simple diagrams used to depict electric circuits
  • resistance is depicted as a zigzag line
  • wires are shown with straight lines
  • a battery is represented with a set of short and long parallel lines

external image image002.jpg
http://www.quantumtheatre.co.uk/QS2%20Webtest_files/image002.jpg


Example of a drawing of a circuit and a schematic diagram of the same circuit. The three sets of short and long parallel lines represent the battery, the straight lines depict the wires, and the zigzag lines are the light bulbs
Example of a drawing of a circuit and a schematic diagram of the same circuit. The three sets of short and long parallel lines represent the battery, the straight lines depict the wires, and the zigzag lines are the light bulbs


http://www.physicsclassroom.com/class/circuits/u9l4a4.gif
  • schematic diagrams are just a simpler way of depicting a drawing of a circuit just without the detailed pictures
  • in the above picture, you can see how much easier it is to use a schematic drawing to show the same exact set up of a circuit

Example of a schematic diagram for a parallel circuit
Example of a schematic diagram for a parallel circuit

http://img.tfd.com/mgh/cep/thumb/Schematic-of-a-parallel-circuit.jpg
Example of a schematic diagram for a series circuit
Example of a schematic diagram for a series circuit

http://www.faqs.org/docs/electric/Exper/05083.png

35.6 Combining Resistors in a Compound Circuit


Example of equalent resistance in parallel:1/4Ω+1/4Ω = 2Ω. 1/6Ω+1/6Ω = 3Ω. 1/12Ω+1/12Ω = 6Ω.
Example of equalent resistance in parallel:1/4Ω+1/4Ω = 2Ω. 1/6Ω+1/6Ω = 3Ω. 1/12Ω+1/12Ω = 6Ω.

http://www.physicsclassroom.com/Class/circuits/u9l4d4.gif
  • the equivalent resistance is the value of the single resistor that would comprise the same load to the battery of pwoer source
  • the EQ for a pair of equal resistors in parallel is half the value of either resistor
    • 1 / RT = 1 / R1 + 1 / R2 + 1 / R3 (+ 1/R4... )
    • this is so because the current has twice the path width
    • if 2 or more resistors are in parallel then the inverses of the total resistance is the sum of the inverses of the individual resistances
    • combine resistors in parallel by halving
  • the EQ for a pair of equal resistors in a series is double the value of either resistor
    • RT = R1 + R2 + R3 (+ R4...)
    • combine resistors in a series by adding
    • with two or more resistors, the total resistance is the sum of the individual resistances
Equivalent Resistance equation for a series circuit
Equivalent Resistance equation for a series circuit

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http://www.riedon.com/us/templates/riedon_internal/calc/Series.jpg
Example of equivalent resistance in series: 6Ω+6Ω = 12Ω. 6Ω+6Ω+6Ω = 18Ω. 6Ω+6Ω+6Ω+6Ω = 24Ω.
Example of equivalent resistance in series: 6Ω+6Ω = 12Ω. 6Ω+6Ω+6Ω = 18Ω. 6Ω+6Ω+6Ω+6Ω = 24Ω.

http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c2.gif

35. 7 Parallel Circuits and Overloading


  • as more devices are added to a parallel circuit the total current will increase, possibly to more than the wires are able to safely handle
  • fuses and circuit breakers are used to limit the total current in a circuit by opening the circuit if they see more than some preset limit